Field of the Invention
The invention is directed to an electron beam generator composed of an electron-emitting cathode, of an anode with an anode bore for the passage of the electron beam along a beam axis, and of a Wehnelt electrode for controlling the electron beam. The cathode, the anode and Wehnelt electrode are arranged in a vacuum chamber.
An electron beam generator is utilized in electron beam devices for processing material, for example in electron beam engraving devices for engraving printing forms for rotogravure in reproduction technology or for engraving surface structures on textured drums with which thin sheets are processed in rolling mills for improving quality.
An electron beam engraving unit for printing forms or textured drums is disclosed, for example, by DE-A40 32 918. The electron beam engraving the is essentially composed of a first vacuum chamber in which an electron beam generator and an electron-optical control system for the electron beam are accommodated, and of a second vacuum chamber in which the printing form or the textured drum to be engraved with the electron beam is located. The electron beam generator is essentially composed of a cathode, of an anode with an anode bore and of a Wehnelt electrode. The electrons that produce the electron beam are emitted by the heated cathode, which lies at high-voltage potential, and are accelerated in the direction toward the anode that, for example, has zero potential. The electron beam is supplied to the electron-optical control system through the anode bore and then passes through an opening in the first vacuum chamber and enters the second vacuum chamber as a working chamber in order to process the printing form or the textured drum.
Given electron beam devices such as, for example, electron beam engraving units that are operated with high-voltage in the kV range and in a high vacuum, ions that arise due to ionization of residual gas molecules occur in the electron beam generator. These ions are accelerated onto the cathode and destroy the cathode by eroding material. It is particularly ions that derive from the acceleration space and are formed immediately in front of the cathode that participate in the destruction of the cathode.
The destruction of the cathode by the ion bombardment makes frequent maintenance work at the electron beam generator necessary, the destroyed cathode having to be thereby replaced by a new cathode. The maintenance jobs disadvantageously lead to standstill times of the electron beams devices. Over and above this, the active emission area is reduced in size due to the erosion of material from the cathode caused by the ion bombardment, and the characteristic current density profile of the electron beam is modified in a disadvantageous way.